Method and nut for preloading ball screw assemblies and method of manufacture of the preload nut member

ABSTRACT

An improved method and nut for preloading ball screw assemblies in which the preloading is accomplished by axially offsetting segments of the internal helical grooves of the nut member, the method comprising permanently deforming an intermediate portion of the nut member so as to cause an axial shift of the connected nut member portion and thus result in axially offset helical groove segments in these portions. The nut member is configured to accommodate readily large permanent deformations. This deformation is produced in the specific process described by rolling a peripheral groove into the nut member.

United States Patent Better et al. 1 Mar. 27, 1973 [54] METHOD AND NUTFoR PRELOADING 2,567,483 9/1951 Hotine ..14/44\ x BALL SCREW ASSEMBLIESAND 3,023,797 3/1962 Greene ..74/44l X METHOD OF MANUFACTURE OF THEPRELOAD NUT MEMBER Inventors: Bernard R. Better; Edward Hain,

both of Chicago, Ill.

Assignee: The Bendix Corporation, Southfield, Mich.

Filed: Mar. 23, 1971 Appl. No.: 127,213

US. Cl ..74/459, 74/409 Int. Cl. ..F16h 55/22, Fl6h 55/ 1 8 Field ofSearch ..74/44l, 424.8 A, 409, 89.15

References Cited UNITED STATES PATENTS 6/1953 Baker ..74/44lX 6/1956Wetzel .;....74/441X Primary Examiner-Leonard H. Ger-in Attorney-John R.Benefiel and Plante, Hartz, Smith & Thompson ABSTRACT An improved methodand nut for preloading ball screw assemblies in which the preloading isaccomplished by axially offsetting segments of the internal helicalgrooves of the nut member, the method comprising permanently deformingan intermediate portion of the nut member so as to cause an axial shiftof the connected nut member portion and thus result in axially offsethelical groove segments in these portions; The nut member is configuredto accommodate readily large permanent deformations. This deformation isproduced in the specific process described by rolling a peripheralgroove into the nut member.

12 Claims, 7 Drawing Figures PATENTEUHARZYIQYS SHEU 10F 2 FIG. I

STEP I STEP 2 INVENTORS BERNARD R. BETTER EDWARD HAIN KBMM ATTORNEYPATEr-flfinumzmn ,722,312

sum 2 or 2 FIGS {17 STEP 5 Z INVENTOES BERNARD R. BETTER EDWARD HAIN 9[307M ATTORNEY BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention concerns ball screw assemblies and more particularly isconcerned with an improved method of preloading ball screw assemblies.

2. Description of the Prior Art Ball screw assemblies are in widespreaduse in precision applications, such as in positioning devices fornumerically controlled machine tools in which backlash must be at anabsolute minimum in either direction of travel, and in which the axialspring rate should be as stiff as practicable.

The most common way of obtaining these results is by preloading the nutmember, an approach which is very well known in the art. Preloadingconsists of axially offsetting helical ball groove turns in the nutmember relative each other and with respect to the screw member so thatthe balls tend to engage opposite groove walls in the respective grooveturns to thus eliminate backlash and stifi'en the spring rate of theassembly (since more balls must be deflected at any mo ment, butprimarily since the spring rate of a ball screw assemblycharacteristically rises sharply over a certain level of axial loading).

This axial offsetting has been accomplished basically in two ways:

(1) Grinding the nut helical groove so that lead of some or a group ofturns is offset from others to thus cause the balls to ramp up on theoffset thread turns. This approach has the advantage of relatively lowcost over other approaches, but inasmuch as it is usually desirable toadjust the preload level for particular applications or to compensatefor loss of preload through wear, this approach has the disadvantage ofbeing difficult to service since these devices must be disassembled anddifferent sets of balls installed. For this same reason, the adjustedpreload level is not easily controlled with any degree of precision. Inaddition, control of the contact angle is lost in using this method ofadjustment which can in turn lead to reduced efficiency and loadcapacity.

Furthermore, the forming of the grooves in two steps may be required,i.e., the offset helical grooves are formed in two separate formingstages and this renders difficult the matching of the geometry of thetwo sets of grooves so formed which is desirable for maximum smoothnessand to minimize differences in forward and reverse drag torques.

Alternatively, relatively complex single pass machining set ups andtechniques are needed which do not alleviate the problems describedabove.

(2) Use of double nuts which have helical groove forms axially offsetfrom each other to create the preload. The nuts are usually adjustableaxially with respect to each other to thus provide for convenientadjustment of the preload level, but the use of two nuts creates highmanufacturing costs since close tolerance machining is necessary inorder to match groove geometry (as each nut is machined separately) aswell as concentricity, squareness of the abutting faces, etc., or elsecompensating devices must be used. In addition,

the nuts must be accurately located by dowelling, etc., during assembly,and even then the nuts may shift radially with respect to each otherunder loading.

Thus it is an object of the present invention to provide an arrangementfor preloading ball screw assem blies without requiring excessivemanufacturing costs, and/or difficulties in servicing, or resulting indetrimental performance.

It is a further object to provide a method of manufacturing a nut forpreload ball screw assemblies which yields maximum smoothness anduniformity of performance, but does not require close tolerance machinmgprocesses.

SUMMARY OF THE INVENTION These objects and others which will becomeapparent upon a reading of the following specification and claims isaccomplished by permanently deforming an intermediate portion of the nutmember so as to cause an axial shift of the connected nut portions andan offsetting of the helical groove form segments therein. In thespecific process described, this permanent deformation is created byrolling a peripheral groove into the intermediate nut portion.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-5 are representations of a nutmember'undergoing the various steps of manufacture according to themethod of the present invention.

FIG. 6 shows the nut member in section undergoing the rolling operationdepicted in FIG. 5.

FIG. 7 depicts a complete ball screw assembly undergoing a preloadingoperation according to the present invention.

DETAILED DESCRIPTION In the following detailed description, certainspecific terminology will be utilized and a specific embodiment will bedescribed in order to provide a full and complete understanding of theinvention, but it is to be understood that the invention is not to be solimited, as many variations are possible without departing from thespirit of the present invention.

Referring to the drawing and particularly to FIGS. 1-5, a process ofmanufacturing a ball screw nut is depicted which incorporates theprinciple of the present invention. In step I, a nut blank MI is roughedout having an internal bore 12 formed therein.

in step 2, the internal helical groove form 14 is formed as byconventional tapping, single point thread chasing or rough grindingprocesses, but preferably with a final, continuous machined pass as by afinish grind to insure maximum uniformity of all segments of the grooveform 14 through the length of the nut blank 10. r

In step 3, the rest of the nut geometry is machined into the blank itsuch as O.D., ball return openings 16, etc. In addition, a peripheralinterior undercut 18 is formed in anintermediate portion of the nut 10,producing a circumferential section of reduced thickness 20,alternatively, or in addition to this interior undercut, an externalrecess (not shown) could be utilized to obtain the desired thickness.

In step 4, a guiding groove 22 is formed about the exterior periphery inthe section 20 to yield a section 24 of still further reduced thickness.This guide groove 22 could be located in the periphery or the optionalexternal recess noted above.

In step 5, the deforming step according to the present invention isperformed. In this step, a wedge-shaped forming wheel 26 shapedcomplementarily to the guide groove 22 is rotated in the guide groove 22while being advanced gradually radially inward as indicated, and whilethe nut is held thereagainst by means of a pair of rollers 28 (FIG. 2).This results in an inward permanent deformation of the section 24 asshown, which in turn, it has been found, results in relative axialshifting of the nut member portions 30 and 32 connected by thesesections to produce the axially offsetting of the internal groove formof nut portions 30 and 32.

The extent of axial shifting is, within limits, increased by furtherinward deformation of the intermediate portion until the desired offsetis attained with the undercut 18 allowing relatively great inwarddeformation.

Upon assembly of the ball screw, the offset internal helical grooveturns create a fixed preload interference with the balls.

To accomplish the rolling operation described in step 5, a conventionalpipe cutter may be modified by substituting the wedge-shaped wheel 26for the cutting wheel to conveniently provide a simple tool for thispurpose, while a pair of cylindrical support rollers 28 will minimizeany build-up of material around the edges of the groove 22.

termediate portion such as the use of swaging equipment. Furthermore,the nut configuration utilizing the undercut and the guide groove whileproviding the desirable advantages of increased precision and extent oftravel in the axial ofiset movement may be varied or omitted. Inconnection with this, axial displacements manner, or the unit may bedisassembled and the nut again deformed in the described manner.

Conversely, a uniform endwise pressure could be applied to the nut tolessen the offset and thus the preload.

As an alternative, the deformation process may be carried out with theunit assembled as depicted in FIG. 3, rolling the intermediate sectionuntil the desired preload level has been attained.

It shouldbe noted that inasmuch as the intermediate section musttransmit the axial forces in certain loading situations, this sectionshould be sized so as to be relatively stiff with respect tothestiffness of theball-nut combination so that the overall stiffness ofthe assembly is not compromised.

It can readily be appreciated that this method provides an extremelysimple and convenient means for creating the preload without theaforesaid disadvantages experienced in prior art approaches.

In typical situations, axial displacements up to 0.023 inches wereattained without distortion or loss of trueness of the connected nutportions, without loss of parallelism of the connected nut portionfaces, and since the deformation is limited to the intermediate section,no distortion of groove geometry is incurred by the operation. 1

It should be notedthat there are many alternative methods foraccomplishing the deformation of the inon the order of 0.006 inch wereattained without the undercut configuration.

What is claimed is:

1. A method of axially offsetting portions of a helically grooved membercomprising:

permanently deforming an intermediate section of saldmember so as tocause a relative axial shift in position of said grooved portions.

2. The method of claim 1 wherein in said deforming step said member ispermanently deformed radially inwardly about its periphery at saidintermediate section.

3. A method of preloading a ball screw assembly having a screw member, anut member and a plurality of recirculating balls comprising the step ofpermanently deforming an intermediate section of said nut so as to causea relative axial shift in position of the nut portions connected therebysufficient to preload said balls in said ball screw assembly.

4. The method of claim 3 wherein said section is permanently deformedradially inwardly about its periphery.

5. The method of claim 4 wherein said deforming step is carried byrolling said section.

6. The method of claim 3 wherein said section of said nut is permanentlydeformed prior to assembly of said ball screw.

7. The method of claim 3 wherein said ball screw is. assembled with saidrecirculating balls in place prior to permanently deforming said nutsection.

8. A nut comprising a member having an internally helically grooved boretherethrough and an intermediate recess extending about the bore andseparating the bore into two portions, and further including an externalgroove formed about said member aligned with said recess, whereby saidnut member may be permanently deformed radially inwardly to cause axialshifting of said portions.

9. A method of manufacturing a nut member which has aligned internallyhelically grooved portions axially offset from each other comprising thesteps of:

forming a. helical groove through a bore in said nut member;

permanently deforming said nut member radially inwardly about itsperiphery at a section intermediate said helical groove, wherebyportions of said helical groove connected by said section are axiallyoffset from each other.

10. The method of claim 9 wherein said deforming step is carried out byrolling a groove in the periphery of said nut member.

11. The method of claim 10 wherein prior to said rolling step anexterior groove is machined said member about the periphery of saidintermediate section to guide said rolling operation.

12. The method of claim 9 wherein prior to deforming said section anundercut is formed in said bore at said intermediate section.

1. A method of axially offsetting portions of a helically grooved member comprising: permanently deforming an intermediate section of saId member so as to cause a relative axial shift in position of said grooved portions.
 2. The method of claim 1 wherein in said deforming step said member is permanently deformed radially inwardly about its periphery at said intermediate section.
 3. A method of preloading a ball screw assembly having a screw member, a nut member and a plurality of recirculating balls comprising the step of permanently deforming an intermediate section of said nut so as to cause a relative axial shift in position of the nut portions connected thereby sufficient to preload said balls in said ball screw assembly.
 4. The method of claim 3 wherein said section is permanently deformed radially inwardly about its periphery.
 5. The method of claim 4 wherein said deforming step is carried by rolling said section.
 6. The method of claim 3 wherein said section of said nut is permanently deformed prior to assembly of said ball screw.
 7. The method of claim 3 wherein said ball screw is assembled with said recirCulating balls in place prior to permanently deforming said nut section.
 8. A nut comprising a member having an internally helically grooved bore therethrough and an intermediate recess extending about the bore and separating the bore into two portions, and further including an external groove formed about said member aligned with said recess, whereby said nut member may be permanently deformed radially inwardly to cause axial shifting of said portions.
 9. A method of manufacturing a nut member which has aligned internally helically grooved portions axially offset from each other comprising the steps of: forming a helical groove through a bore in said nut member; permanently deforming said nut member radially inwardly about its periphery at a section intermediate said helical groove, whereby portions of said helical groove connected by said section are axially offset from each other.
 10. The method of claim 9 wherein said deforming step is carried out by rolling a groove in the periphery of said nut member.
 11. The method of claim 10 wherein prior to said rolling step an exterior groove is machined said member about the periphery of said intermediate section to guide said rolling operation.
 12. The method of claim 9 wherein prior to deforming said section an undercut is formed in said bore at said intermediate section. 